1. Technical Field
The present disclosure relates to an integrated multilayer magnetoresistive sensor and to the manufacturing method thereof, more in particular to a magnetic-field sensor comprising anisotropic magnetoresistive elements.
2. Description of the Related Art
Magnetic-field sensors of an AMR (anisotropic magnetoresistance) type are used in a plurality of applications and systems, for example, in compasses, in systems for detecting ferromagnetic materials, in the detection of currents and in a wide range of other applications, thanks to their capacity for detecting natural magnetic fields (for example, the Earth's magnetic field) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines traversed by electric current).
In a known way, the phenomenon of anisotropic magnetoresistivity occurs within particular ferromagnetic materials, which, when subjected to an external magnetic field, undergo a variation of resistivity according to the characteristics of the applied magnetic field. Usually, said materials are shaped in thin strips so as to form resistive elements and the resistive elements thus formed are electrically connected together to form a bridge structure (typically a Wheatstone bridge).
It is moreover known to obtain AMR magnetic sensors with standard techniques of micromachining of semiconductors, as described for example, in the document No. U.S. Pat. No. 4,847,584. In particular, each magnetoresistive element can be formed by a film of magnetoresistive material, such as for example, permalloy (a ferromagnetic alloy containing iron and nickel), deposited to form a thin strip on a substrate of semiconductor material, for example, silicon.
When an electric current is made to flow through a magnetoresistive element, the angle θ between the direction of magnetization of said magnetoresistive element and the direction of the flow of the current affects the effective value of resistivity of said magnetoresistive element, so that, as the value of the angle θ varies, the value of electrical resistance varies (in detail, said variation follows a law of the cos2θ type).
For example, a direction of magnetization parallel to the direction of the flow of current results in a maximum value of resistance to the passage of current through the magnetoresistive element, whereas a direction of magnetization orthogonal to the direction of the flow of current results in a minimum value of resistance to the passage of current through the magnetoresistive element.
AMR magnetic sensors moreover include a plurality of coils integrated in said sensors (typically two coils), referred to as “set/reset strap” and “offset strap” and designed to generate, when traversed by a current of appropriate value, a magnetic field that couples in a direction perpendicular to the direction of detection of the sensors and, respectively, in the direction of detection of the sensors; in this regard, see for example, the document No. U.S. Pat. No. 5,247,278.
The set/reset strap has the function of varying, by alternating it, the sense of magnetization of the magnetoresistive elements in a first predefined direction (the so-called “easy axis” or EA). In use, the variation of the sense of magnetization is obtained by applying to the magnetoresistive element, via the set/reset strap, a magnetic field of appropriate value for a short period of time, such as to force arbitrarily the sense of the magnetic dipoles of the magnetoresistive element in the first predefined direction (set operation) and, subsequently, by applying to the magnetoresistive element a second magnetic field, like the previous one but with opposite sense, so as to force the sense of the magnetic dipoles of the magnetoresistive element once again in the first predefined direction, but with opposite sense (reset operation).
The set and reset operations have the function of bringing each magnetoresistive element of the AMR sensor into a respective single-domain state before operating the AMR sensor, for example, in order to carry out operations of sensing of an external magnetic field. The set and reset operations are used in so far as only in the single-domain state the fundamental properties of linearity, sensitivity and stability of the magnetoresistive elements are controlled and repeatable. The aforementioned set and reset operations are known and described in detail for example, in the document No. U.S. Pat. No. 5,247,278.
The offset strap is normally used for operations of compensation of the offset present in the AMR sensor (on account of mismatch in the values of the corresponding electrical components), self-test operations and/or operations of calibration of the AMR sensor. In particular, the value of the electrical quantities at output from the AMR sensor is, in the presence of the offset strap, a function both of the external magnetic field to be sensed and of the magnetic field generated as a result of a current circulating in the offset strap. The offset strap is formed by turns of conductive material, for example, metal, arranged on the same substrate as that on which the magnetoresistive elements of the sensor and the set/reset strap are provided (in different metal levels) and is electrically insulated from and set in the proximity of, said magnetoresistive elements. The magnetic field generated by the offset strap is such as to force partially the orientation of the magnetic dipoles of each magnetoresistive element in a second predefined direction (the so-called “hard axis” or HA), orthogonal to the first predefined direction.
FIG. 1 shows, in top plan view, a layout exemplifying an integrated magnetic-field sensor 1 of a known type and comprising a plurality of magnetoresistive elements, connected together so as to form a Wheatstone bridge, for example, as described in the document No. U.S. Pat. No. 5,247,278 and the document No. U.S. Pat. No. 5,952,825 and manufactured as described, for example, in the document No. U.S. Pat. No. 4,847,584.
More in particular, each magnetoresistive element has a structure of the barber-pole type. The barber-pole structure for magnetoresistive elements is known. In this case, each magnetoresistive element is formed by a strip made of magnetoresistive material (typically an NiFe alloy), arranged on which, in direct electrical contact, are conductive elements with high electrical conductivity (for example, made of aluminum, copper, silver, or gold).
Said conductive elements extend in a direction transverse to the axis of spontaneous magnetization of the magnetoresistive strip; in particular, they are inclined by a certain angle α (typically, α=45°) with respect to the axis of spontaneous magnetization of the magnetoresistive strip.
The magnetic-field sensor 1 is formed on a semiconductor substrate 2 by micromachining processes of a known type. Four magnetoresistive elements 4, 6, 8 and 10, in the form of strips of ferromagnetic material (for example, deposited thin films comprising an Ni/Fe alloy), in barber-pole configuration, are electrically connected together to form a Wheatstone bridge. For each magnetoresistive element 4, 6, 8, 10, the magnetoresistive strips that form it are connected in series to one another. With reference to FIG. 1, the magnetoresistive elements 4, 6, 8, 10 are interconnected and connected to terminals (or pads) 21, 22, 23, 24 and 25. The terminal 21 is connected to the magnetoresistive element 4 by a conductive path 11 and the magnetoresistive element 4 is connected to the magnetoresistive element 6 by a conductive portion 16. The conductive portion 16 is electrically connected to the terminal 22 by a respective conductive path 12. The magnetoresistive element 6 is then connected to the magnetoresistive element 10 by a conductive portion 18 and the conductive portion 18 is electrically connected to the terminal 23 by a respective conductive path 13. The magnetoresistive element 10 is interconnected with the magnetoresistive element 8 by a conductive portion 17 and the conductive portion 17 is electrically connected to the terminal 24 by a respective conductive path 14. The terminal 25 is connected to the magnetoresistive element 8 by a conductive portion 15.
In this way, a resistive Wheatstone-bridge structure is formed, which provides a magnetic-field sensor 1 sensitive to components of magnetic field having a direction perpendicular to the strips of ferromagnetic material that form the magnetoresistive elements 4, 6, 8, 10. The terminal 21 is connected to the terminal 25 to form a common terminal so as to connect electrically the magnetoresistive element 4 and the magnetoresistive element 8.
In use, an input voltage Vin is applied between the terminal 22 and the terminal 24. Reading of the output voltage Vout is made between the terminal 21 (common to the terminal 25) and the terminal 23.
With reference to FIG. 1, the magnetic-field sensor 1 further comprises a first strip of electrical conductive material, which extends over the substrate 2 and is insulated from the latter by a layer of dielectric material (not illustrated in detail in the figures). Said first strip of electrical conductive material forms a first winding 19, of a planar type, which extends in a plane parallel to the plane in which the magnetoresistive elements 4, 6, 8, 10 and is electrically insulated from the magnetoresistive elements 4, 6, 8, 10, lie.
The magnetic-field sensor 1 further comprises a second strip of electrical conductive material, which extends over the substrate 2 and is insulated from the latter and from the first winding 19 by a layer of dielectric material (not illustrated in detail in the figures). Said second strip of electrical conductive material forms a second winding 20, of a planar type, which extends between a terminal 20a and a terminal 20b in a plane parallel to the plane in which the magnetoresistive elements 4, 6, 8, 10 and the first winding 19 lie and is electrically insulated from the magnetoresistive elements 4, 6, 8, 10 and from the first winding 19.
The first winding 19 is used to generate a magnetic field having a known intensity interacting with the magnetic-field sensor 1, with purposes of biasing, calibration and/or compensation of possible offsets due to the presence of undesirable external magnetic fields. In the latter case, the effect of the magnetic field generated by the first winding 19 on the output signal Vout of the magnetic-field sensor 1 is that of balancing the output signal due exclusively to the undesirable external magnetic field in order to generate a zero output signal.
In use, when the first winding 19 is traversed by electric current, a magnetic field is generated, the lines of force of which have a component in the plane in which the magnetoresistive elements 4, 6, 8, 10 lie, in particular, in a direction parallel to the direction of sensitivity of the magnetoresistive elements 4, 6, 8, 10.
On account of the spread of the process of production of the magnetoresistive elements 4, 6, 8, 10, said magnetoresistive elements 4, 6, 8, 10 can present structural characteristics different from one another. This generates an offset signal Voff, superimposed on the useful output signal Vs, intrinsic to the magnetic-field sensor 1, which causes a reduction of the sensitivity of the magnetic-field sensor 1 during use. Said offset signal Voff can be eliminated by appropriately operating the second winding 20. In greater detail, during use, current pulses are made to flow in the second winding 20 with directions opposite to one another (by appropriately biasing the terminals 20a and 20b of the second winding) in such a way as to generate respective magnetic fields defined by respective field lines having senses opposite to one another. Said magnetic fields have an intensity such as to re-orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, according to the field lines generated, in particular with a sense defined by the sense of the lines of the magnetic field generated.
Following upon a first current pulse (referred to as set pulse) through the second winding 20, a first magnetic field HS1 is generated such as to orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 in a first sense.
Following upon a second current pulse (referred to as reset pulse) through the second winding 20, a second magnetic field HS2 (of intensity, for example, equal to that of the first magnetic field HS1) is generated such as to orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 in a second sense.
The measurement made after the set step (signal taken from an output of the bridge) is equal to Voff+Vs, whereas after reset we find Voff−Vs. By subtracting the two measurements, the offset cancels out, thus obtaining 2Vs.
The AMR sensor described with reference to FIG. 1 hence requires, to be operated correctly, at least two straps (the set/reset strap and the offset strap).
The presence of a strap for the set/reset operations and of a strap for the operations of offset compensation complicates the process of manufacture of the AMR sensor and increases the production cost thereof. Moreover, also the dimensions of the sensor thus manufactured are not optimized.
The aforementioned disadvantages are even more evident in the case of two-axis sensor or three-axis sensor, designed to measure magnetic fields acting along two or, respectively, three axes. In this case, in fact, it would be necessary to provide at least one magnetoresistive element for each measuring axis and to provide a set/reset strap and an offset-compensation strap for each magnetoresistive element. It is evident that the occupation of area and manufacturing difficulties are, in this case, an important issue.